EVOH resin can exhibit superior excellent gas-barrier performance such as oxygen-barrier performance due to the fact that hydrogen bond is formed between hydroxyl groups in the molecular chain of the EVOH resin to form a crystal part which can prevent oxygen from intruding. For this gas barrier performance, EVOH resin is molded into a film or sheet used as a wrapping material for food, pharmaceuticals, industrial medicine and agrochemicals, or container such as bottle.
In general, molding EVOH resin into film, sheet, bottle, cup, tube, or pipe is performed by melt-molding. The resulting molded article is processed for actual use. Therefore moldability and processability are important properties for EVOH resin.
Since EVOH resin has a relatively active hydroxyl group in its molecule, the EVOH resin in a molten state under high temperatures may be oxidized or crosslinked to generate a deteriorated matter because of heat. It is known that the deteriorated matter may result in gel or bump which brings out fish eye associated with a defect of the molded article.
A patent document 1 suggests an EVOH resin composition comprising EVOH resin and a conjugated polyene, as an EVOH resin composition which is reduced in the occurrence of fish eyes, gel and/or bump when melt-molding.
According to the method disclosed in the patent document 1, a conjugated polyene having a boiling point of 20° C. or more is added to the copolymer of vinyl acetate or a like monomer, followed by saponifying the obtained copolymer. The obtained EVOH resin composition comprises an EVOH resin and the conjugated polyene wherein the concentration of the conjugated polyene is from 0.00002 to 1 wt % (i.e. 0.02 ppm to 10000 ppm), preferably 0.0001 to 0.2 wt % (i.e. 1 ppm to 2000 ppm) (paragraph 0034).
The patent document 1 describes that a polyene compound still remained in a final product after saponification was an origin of odor or exudation or the like, and therefore a relatively high residual amount is not good for wrapping material such as container. The patent document 1 also describes that a polyene compound which is readily removable by rinsing with water is recommended (paragraph 0011).
According to Example 1 of the patent document 1, EVOH resin was obtained by polymerizing vinyl acetate under a pressurized ethylene, thereafter adding conjugated polyene and removing ethylene, followed by saponification. Thus obtained EVOH resin particles were rinsed with a large amount of water to produce desired EVOH resin particles.
The patent document 1 shows in Table 1 that the EVOH resin particles could be formed into a film with less gel bumps each having a size of 100 μm or more, specifically from 3 to 10 of spots per area of 100 cm2.
A patent document 2 also discloses a method of producing a vinyl alcohol-based polymer capable of reducing not only occurrence offish eyes in molding but also coloration and odor. The method comprises polymerizing vinyl acetate-based monomer, removing unreacted vinyl acetate from a distillation tower after the polymerization, and feeding a polymerization inhibitor which is at least one of N,N-dialkyl hydroxyl amine, styrene derivative, hydroquinone derivative, quinone derivative, piperidine derivative, or conjugated polyene.
[Patent document 1] JP H9-71620A
[Patent document 2] JP2003-89706A